<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script type="text/javascript">
        function draw(){
            var canvas = document.getElementById("canvas");
            if (canvas.getContext){
                var ctx = canvas.getContext('2d');
                ctx.fillStyle = "rgb(200,0,0)";
                ctx.fillRect (10, 10, 55, 50);

                ctx.fillStyle = "rgba(0, 0, 200, 0.5)";
                ctx.fillRect (30, 30, 55, 50);
            }else{alert();}
        }
    </script>
</head>
<body>
<canvas id="canvas" width="150" height="150" style="border: 1px solid"></canvas>
//
</body>
</html>